1. Field of the Invention
The present invention relates to a lens system for a deflected light beam and more particularly to a lens system including an f.theta. lens and a corrective lens.
2. Discussion of the Background
Light beam scanners for deflecting a monochromatic light beam such as a laser beam or a quasimonochromatic light beam such as a light beam emitted from a monochromatic fluorescent lamp are heretofore widely known in relation to optical printers and information readout devices.
In such light beam scanners, a lens system for a deflected light beam is disposed between a deflection point about which the light beam is deflected and a surface to be scanned by the light beam. The lens system is generally composed of an f.theta. lens and a corrective lens.
The f.theta. lens serves to correct the light path of the light beam deflected at a constant speed for equalizing the speed of movement of a beam spot on the surface being scanned. The direction in which the scanning beam spot moves on the surface being scanned is referred to as a main scanning direction, whereas the direction normal to the main scanning direction is referred to as an auxiliary scanning direction.
The corrective lens cooperates with the f.theta. lens in constituting a surface irregularity correcting optical system, and is generally in the form of a cylindrical lens. The term "surface irregularity" used herein typically means an inclination of a mirror surface of a rotating polygonal mirror out of parallel with adjacent mirror surfaces, i.e., the axis of the rotating polygonal mirror. Surface irregularity correction means the prevention of the position of the scanning beam spot from being subjected to fluctuations in the auxiliary scanning direction which would otherwise arise from surface irregularities, i.e., variations in the direction of the reflecting mirror surfaces of a rotating polygonal mirror or the disc surfaces of a hologram grating disc. The f.theta. lens and the corrective lens are used to keep the deflection point and the position of the main scanning line in conjugate relation to each other in the auxiliary scanning direction.
Conventional f.theta. lenses that have been prevalent in the art have relatively small effective deflection angles of 50 degrees or less. However, there has recently been proposed the use of an f.theta. lens having an effective deflection angle of 50 degrees or more (see, for example, Japanese Laid-Open Patent Publication No. 59(1984)-147316).
It is known that an increase in the effective deflection angle of an f.theta. lens would result in an increase in the curvature of field in the auxiliary scanning direction. Therefore, there has been a demand for a lens system for a deflected light beam which has a large effective deflection angle with the curvature of field being compensated.